Audio signal | EPFL Graph Search

An audio signal is a representation of sound, typically using either a changing level of electrical voltage for analog signals, or a series of binary numbers for digital signals. Audio signals have frequencies in the audio frequency range of roughly 20 to 20,000 Hz, which corresponds to the lower and upper limits of human hearing. Audio signals may be synthesized directly, or may originate at a transducer such as a microphone, musical instrument pickup, phonograph cartridge, or tape head. Loudspeakers or headphones convert an electrical audio signal back into sound. Digital audio systems represent audio signals in a variety of digital formats. An audio channel or audio track is an audio signal communications channel in a storage device or mixing console. It is used in operations such as multi-track recording and sound reinforcement. Signal flow Signal flow is the path an audio signal will take from source to the speaker or recording device. Signal flow may be shor

Semi-supervised Extraction of Audio-Visual Sources

Patricia Calatayud Martinez

This report presents a semi-supervised method to jointly extract audio-visual sources from a scene. It consist of applying a supervised method to segment the video signal followed by an automatic process to properly separate the audio track. This approach starts with the user interaction to select the audio-visual target we want to cut. This labels the problem into two parts. One is the foreground, the object we aim to extract. The background is the remaining content which sets the other part. The segmentation method will be performed over the video signal to split its visual content into foreground and background. It is based on describing the video information as a volume. It is composed of pixels and relationships among them which set the features of the problem. This 3D structure can be interpreted as a graph. Therefore the segmentation is achieved by applying a Graph Cuts algorithm which provides the suitable results. In this work the audio separation process is an automatic task. Due to this we need to extract prior information from the audio-visual data available. Therefore a link between audio and video channels is deﬁned by an audio-visual motion map. This is a video sequence which provides the amount of synchronous motion with the audio track. An audio-based video diﬀusion method have been developed to obtain this kind of signal. Applying the video segmentation labelling to the audio-visual motion map we obtain the distribution of the amount motion into foreground and background. According to the range of these values it is possible to asses when the corresponding audio is on/oﬀ. At this point, audio samples of sources can be extract when the audio activity is due to only one. This is the prior information to learn audio models by means of spectral GMM estimation to assess the audio signals. Finally the separation method is applied. It consist of looking for the more suitable couple of states given the mixture spectrum of the total audio track. Therefore we achieve the audio separation goal. In this work we will study in detail all the stages towards the ﬁnal aim of our method, analyzing the performances of our approach.

2010

Audio-Visual Fusion

Anna Llagostera Casanovas

The perception that we have about the world is influenced by elements of diverse nature. Indeed humans tend to integrate information coming from different sensory modalities to better understand their environment. Following this observation, scientists have been trying to combine different research domains. In particular, in joint audio-visual signal processing the information recorded with one or more video-cameras and one or more microphones is combined in order to extract more knowledge about a given scene than when analyzing each modality separately. In this thesis we attempt the fusion of audio and video modalities when considering one video-camera and one microphone. This is the most common configuration in electronic devices such as laptops and cellphones, and it does not require controlled environments such as previously prepared meeting rooms. Even though numerous approaches have been proposed in the last decade, the fusion of audio and video modalities is still an open problem. All the methods in this domain are based on an assumption of synchrony between related events in audio and video channels, i.e. the appearance of a sound is approximately synchronous with the movement of the image structure that has generated it. However, most approaches do not exploit the spatio-temporal consistency that characterizes video signals and, as a result, they assess the synchrony between single pixels and the soundtrack. The results that they obtain are thus sensitive to noise and the coherence between neighboring pixels is not ensured. This thesis presents two novel audio-visual fusion methods which follow completely different strategies to evaluate the synchrony between moving image structures and sounds. Each fusion method is successfully demonstrated on a different application in this domain. Our first audio-visual fusion approach is focused on the modeling of audio and video signals. We propose to decompose each modality into a small set of functions representing the structures that are inherent in the signals. The audio signal is decomposed into a set of atoms representing concentrations of energy in the spectrogram (sounds) and the video signal is concisely represented by a set of image structures evolving through time, i.e. changing their location, size or orientation. As a result, meaningful features can be easily defined for each modality, as the presence of a sound and the movement of a salient image structure. Finally, the fusion step simply evaluates the co-occurrence of these relevant events. This approach is applied to the blind detection and separation of the audio-visual sources that are present in a scene. In contrast, the second method that we propose uses basic features and it is more focused on the fusion strategy that combines them. This approach is based on a nonlinear diffusion procedure that progressively erodes a video sequence and converts it into an audio-visual video sequence, where only the information that is required in applications in the joint audio-visual domain is kept. For this purpose we define a diffusion coefficient that depends on the synchrony between video motion and audio energy and preserves regions moving coherently with the presence of sounds. Thus, the regions that are least diffused are likely to be part of the video modality of the audio-visual source, and the application of this fusion method to the unsupervised extraction of audio-visual objects is straightforward. Unlike many methods in this domain which are specific to speakers, the fusion methods that we present in this thesis are completely general and they can be applied to all kind of audio-visual sources. Furthermore, our analysis is not limited to one source at a time, i.e. all applications can deal with multiple simultaneous sources. Finally, this thesis tackles the audio-visual fusion problem from a novel perspective, by proposing creative fusion methods and techniques borrowed from other domains such as the blind source separation, nonlinear diffusion based on partial differential equations (PDE) and graph cut segmentation.

EPFL2011

Blind audiovisual source separation using sparse representations

Anna Llagostera Casanovas, Gianluca Monaci, Pierre Vandergheynst

In this work we present a method to jointly separate active audio and visual structures on a given mixture. Blind Audiovisual Source Separation is achieved exploiting the coherence between a video Signal and a one-microphone audio track. The efficient representation of audio and video sequences allows to build relationships between correlated structures on both modalities. Video structures exhibiting strong correlations with the audio signal and that are spatially close are grouped using a robust clustering algorithm that can count and localize audiovisual sources. Using such information and exploiting audio-video correlation, audio sources are also localized and separated. To the best of our knowledge this is the first blind audiovisual source separation algorithm conceived to deal with a video sequence and the corresponding mono audio signal.

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